A. K. Jain, A. Ross, and S. Prabhakar, “An introduction to biometric recognition,” IEEE Trans. Circuits Syst. Video Technol., vol. 14, 2004 discloses that the iris of the eye is a near-ideal biometric. Typically an image of an iris is best acquired in a dedicated imaging system that uses infra-red (IR) illumination, typically near infra-red (NIR) in the range 700-900 nm.
WO2016/020147 (Ref: FN-397) discloses an optical system for an image acquisition device comprising a filter comprising a central aperture arranged to transmit both visible and selected near infra-red (NIR) wavelengths and a peripheral aperture arranged to block visible wavelengths and to transmit the NIR wavelengths. An image sensor comprises an array of pixels including pixels sensitive to visible wavelengths and corresponding pixels sensitive to the NIR wavelengths. A lens assembly is axially located between the filter and the image sensor and comprises a plurality of lens elements. The lens elements are arranged to simultaneously focus NIR light received from a given object through central and peripheral apertures of the filter and visible light received from the object through the central aperture onto the sensor surface.
The optical system of WO2016/020147 is shallow enough to be accommodated in a smartphone housing and to enable a user to capture both visible wavelength images for general purpose photography as well as IR images of a user's iris for biometric recognition. However, it is limited in the distance at which it can acquire an in-focus IR image which can be used for biometric recognition.
PCT/EP2016/052395 (Ref: FN-452) discloses an optical system for an image acquisition device comprising an image sensor comprising an array of pixels including pixels sensitive to IR wavelengths for acquiring an image. A lens assembly includes a collecting lens surface with an optical axis, the lens assembly being arranged to focus IR light received from a given object distance on the sensor surface. The lens assembly includes at least a first reflective surface for reflecting collected light along an axis transverse to the optical axis so that a length of the optical system along the optical axis is reduced by comparison to a focal length of the lens assembly.
In PCT/EP2016/052395 in particular, the optical system comprises a dedicated lens for acquiring IR images which is shallow enough to be accommodated within a smartphone housing and which allows a user to authenticate themselves to the smartphone.
It will be appreciated that for any optical system which is to be employed within a smartphone for acquiring IR images of a user's iris, it is desirable to be able to acquire in-focus images of a user's iris across as large a range of distances as possible to allow for maximum variation in the distance at which a user holds a smartphone from their face, but without needing to increase the depth of the smartphone housing to accommodate the optical system.